In ink-jet ink chemistry, the majority of commercial ink-jet inks are water-based. Thus, their constituents are generally water-soluble, as in the case with many dyes, or water dispersible, as in the case with pigments. Furthermore, ink-jet inks have low viscosity (typically 5 cps or less) to accommodate high frequency jetting and firing chamber refill processes common to ink-jet pens.
Polymer-encapsulated pigments of various kinds are known. However, the polymer chemistries of these pigments are typically incompatible or effective for use with thermal ink-jet printheads. Such compositions tend to either agglomerate under the high thermal shear conditions of the pen firing chamber, causing nozzle and ink channel blockages, or have excessive glass transition temperatures that prevent room temperature print film formation. Thus, incorporation of such polymer encapsulated pigments within thermal ink-jet inks can result in pen unreliability or poor print durability colorant performance.
Ink-jet prints are also often known for poor durability when exposed to water or high humidity. This results from the use of water-soluble and water dispersible colorants within the water-based ink. There has been great improvement in the area of water durability of ink-jet inks through incorporation of certain ink-jet compatible latex polymers. When printed as part of an ink-jet ink, a latex component of the ink can form a film on a media surface, entrapping and protecting the colorant within the hydrophobic print film. However, not all colorant will necessarily be protected upon printing, as would be optimal.
Polymers that form durable films are typically made from copolymers having bulk densities on the order of 1.15 g/cm3 or greater, which is appreciably greater than water, the primary component of thermal ink-jet ink. As such, conventional latex particles are normally designed to flocculate so that latex precipitate may be easily shaken or stirred back into dispersion without agglomeration. Such flocculation behavior is well known with latex paints. Unfortunately, these conventional teachings do not address the unique needs of ink-jet printing applications. For example, the micro-channel ink feeds in ink-jet pens are easily clogged with precipitant, particularly when a pen is stored or otherwise unused for prolonged periods of time. Such precipitation is not easily redispersed by pen shaking, as flow constriction prohibits adequate mixing within micro-channels of pen architecture. Additionally, micro-channels used for jetting can house some of the ink over prolonged periods in preparation for firing, and settled latex can cause further constricting of the micro-channels. This can result in ink-jet pen failure due to clogging of the micro-channels. Further, the micron-order settling distances found in the fluid channels of thermal inkjet pens exacerbate the problem.